<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        // 声明 但是不赋值
        var a;
        document.write(a);
        // 不声明
        document.write(b);


    </script>
</body>

</html>